1. Field of the Invention
The present invention relates to a digital signal processing circuit which is suitable for use in digital audio devices and the like.
2. Prior Art
The known digital signal processor (i.e., DSP) is capable of performing a fade-in operation as well as a fade-out operation by performing multiplication, using predetermined coefficients, on digital signals inputted thereto; or the DSP is capable of combining those operations so as to perform cross-fade operations by which an sound effect (or acoustic effect) to be imparted to input signals is varied with respect to time.
FIG. 4 shows a simple configuration for the circuit which performs the cross-fade operations.
In the circuit shown in FIG. 4, an input signal applied to an input terminal is delivered to filters F1 and F2. Output signals of the filters F1 and F2 are respectively supplied to multipliers K1 and K2 in which they are multiplied by respective coefficients. Then, outputs of the multipliers K1 and K2 are added together by an adder ADD, so that a result of addition is outputted through an output terminal.
If the sound effect to be imparted to the input signal is not subjected to time-related variation, the multiplication coefficient used by the multiplier K1 is set at `1`, while the multiplication coefficient used by the multiplier K2 is set at `0`, so that only the output signal of the filter F1 is outputted.
On the other hand, when the sound effect is subjected to time-related variation, a filter coefficient of the filter F2 is renewed; and then the multiplication coefficient of the multiplier K2 is gradually increased up to `1` from `0` while the multiplication coefficient of the multiplier K1 is gradually decreased to `0` from `1`. Those operations will realize the cross-fade operations in the circuit shown in FIG. 4. According to the cross-fade operations, the sound effect to be imparted to the input signals is gradually shifted from a first sound effect, corresponding to a filtering characteristic of the filter F1, to a second sound effect corresponding to a filtering characteristic of the filter F2.
In order to perform arithmetic operations by using a great number of coefficients, the DSP performs convolution operations. Herein, a digital signal of one word is supplied to the DSP in each sampling period; hence, the DSP stores those digital signals in turn. Thus, the DSP performs the convolution operations on a certain amount of digital signals which are stored therein within a certain period of time.
As described above, the DSP inputs a new sample of data (i.e., new digital signal) by each sampling period; hence, the number of the samples of data which are sequentially stored in the DSP is increased in response to a progress in the number of sampling periods to be passed away. The DSP uses a certain number `n` for the number of the samples of data, which are previously stored therein by the current sampling period in order to perform the convolution operations. If the sample of data, which is stored in the DSP at the current sampling period, is represented by a symbol "X.sub.m ", the DSP uses the samples of data, represented by "X.sub.m " to "X.sub.m-n+1 ", for the convolution operations using multiplication coefficients "C.sub.1 " to "C.sub.n " respectively. The convolution operation is expressed by an equation as follows: EQU Y.sub.m =.SIGMA..sup.n.sub.k=1 C.sub.k.X.sub.m-k+1 ( 1)
In the above equation (1), "Y.sub.m " represents a result of the convolution operations performed by the DSP.
The DSP described above is used in a variety of applications in the signal processing such as the filter processing and reverberation-imparting processing.
In order to change the sound effect to be imparted to the digital signals inputted while performing a variety of operational steps for imparting the sound effect, it is necessary to change the multiplication coefficients.
However, if the multiplication coefficients are changed suddenly, the output signals of the DSP are sharply changed, which will cause noises offensive to the ear. In order to avoid such a drawback, the DSP should perform a processing by which the multiplication coefficients are successively renewed. In the DSP conventionally known, such processing is performed by the external control portion such as the CPU.
According to the method of successively changing the multiplication coefficients by the CPU, the CPU should write the renewed coefficients into the DSP many times. Normally, the CPU is designed to perform a variety of processing for the device which contains the DSP. Therefore, it is not preferable for the CPU to take much time for the operation to write the renewed coefficients into the DSP.
In the case of the cross-fade operations, it is not necessary to set the filter coefficients so many times as compared to the multiplication coefficients. Hence, the number of the tasks which are loaded on the CPU may not be increased so much by undertaking the task for setting the coefficients. However, if the sound-effect characteristic is changed from one to another in order to obtain a variety of sound effects while gradually shifting the sound effect from one to another, the noises are easily produced and are incorporated in the output signals of the DSP. Therefore, an improvement of the DSP is demanded.